Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which includes an illumination device and a projection lens. The image of a mask (=reticle) illuminated by way of the illumination device is in this case projected by way of the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the EUV range, e.g. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process.
The EUV light is produced via an EUV light source which is based on a plasma excitation. This EUV light source includes a CO2 laser for producing infrared radiation, the infrared radiation being focussed by way of a focusing optical unit and—as depicted merely schematically in FIG. 6—passing through an opening 601 present in a collector mirror 600 that is embodied as an ellipsoid, and being guided to a target material (e.g. tin droplet) that is produced via a target source and supplied to a plasma ignition position. The infrared radiation heats the target material situated in the plasma ignition position in such a way that the target material transitions into a plasma state and emits EUV radiation (with the plasma being denoted by “602” in FIG. 6). This EUV radiation is focussed onto an intermediate focus by way of the collector mirror 600 and enters a downstream illumination device via this intermediate focus.
In order to avoid contamination of the optical effective surface of the collector mirror 600 with target material (tin in this example), the practice of guiding hydrogen gas (as indicated in FIG. 6) over the collector surface is known. Here, the EUV radiation decomposes the hydrogen molecules into hydrogen radicals which, in turn, chemically combine with the tin, whereupon the arising Sn—H compound can be pumped away. Moreover, as indicated in FIG. 6, hydrogen gas can also be guided directly in the direction of the plasma 602 in order to keep tin (Sn) ions away from the collector surface in the first place.
In order to avoid a loss of reflectivity of the reflective optical components by contaminants entering into the respective optical system, the immediate surroundings of the relevant reflective optical components are charged with an atmosphere made of hydrogen (as a “purge gas”) in other regions of the projection exposure apparatus as well, the atmosphere of hydrogen being intended to prevent the ingress of unwanted contaminants in the immediate surroundings of these reflective optical components.
It can arise in practice in all of the above-described cases that hydrogen radicals enter into the layer system that is present on the substrate of the respective reflective optical component, reach as far as the substrate surface, recombine there to form hydrogen molecules and, by way of a bubble formation (so-called “blister formation”) that accompanies the accumulation of gas phases, lead to a layer detachment and, as a consequence, to a loss of reflectivity or the destruction of the reflective optical element.
Such a scenario is indicated purely schematically in FIG. 7, where “705” in this case denotes a mirror substrate, on which a layer system made of an intermediate layer 710 and a reflection layer system 720 (which in the example includes a multiple layer system made of an alternating succession of molybdenum (Mo) and silicon (Si) layers) are provided. As indicated in FIG. 7, the risk of ingress of the hydrogen radicals is increased in regions in which the reflection layer system 720 is interrupted by scratches, holes or pores, since the barrier effect that is still provided by the multiple layer system is no longer present there.
The issue described above may be particularly grave—as indicated in FIG. 8A and FIG. 8B—in edge regions of the respective optical element or mirror if, at the locations, either (according to FIG. 8A) the reflection layer system 820 that impedes the diffusion of the hydrogen atoms is exposed or no longer present in edge regions (FIG. 8A) or if the reflection layer system is damaged in an edge region 821 by scratches (FIG. 8B), as a consequence of which hydrogen radicals are able to enter and blister formation may occur.
Reference is made merely by way of example to DE 10 2014 216 240 A1, DE 10 2014 222 534 A1, DE 10 2013 102 670 A1, DE 10 2011 077 983 A1, WO 2012/136420 A1 and EP 2 905 637 A1.